The BGS Lexicon of Named Rock Units — Result Details

Sudbury Formation

Formation encompasses fluvial, lacustrine and organic deposits of the pre-diversionary River Thames. Most of the surviving deposits are fluvial gravels, with sedimentary structures indicating deposition by a braided river. Lacustrine silts and clays and organic peats are uncommon. The formation represents the first significant input of far-travelled materials into the Thames river sediments, and is characterised by quartz and quartzite from the Triassic, Carboniferous and Devonian rocks of the West Midlands, Welsh Borderland and possibly southwestern Pennines, and felsic volcanic rocks from northern Wales. The presence of mega-erratics and glacially fractures sand grains indicate glacial erosion in the headwater regions of the river. The fluvial gravels occupy terrace levels and the members are defined on the basis of altitude and pebble clast content. The members comprise bodies of cross-bedded and massive, moderately sorted sand and gravel. The aggradations are generally entrenched into bedrock with a difference in surface elevation of at least 10m.

Definition of Lower Boundary:

Unconformable on bedrock: Cretaceous, Palaeogene and Crag Group. Differentiation from the Crag Group can be difficult where the latter is reworked, and the distinction must be made on the basis of sedimentological structures: the Crag Group is marine, the Kesgrave Catchment Subgroup is fluvial.

Definition of Upper Boundary:

Commonly overlain by Mid Pleistocene glacigenic deposits. Upper boundary may be difficult to determine where overlain by glaciofluvial sand and gravel, but the presence of more angular clasts, chalk, and poorer sorting in the latter is usually helpful.

Thickness:

c.18m. Individual terrace aggradations typically 5-10m thick.

Geographical Limits:

Formation is restricted to the Thames Valley, Essex and Suffolk. It does not extend into Norfolk (Hamblin and Moorlock, 1995).

Whiteman, C A and Rose, J. 1992. Thames river sediments of the British Early and Middle Pleistocene. Quarterly Science Reviews, Vol.11, 363-375.

Whiteman, C A. 1992. The palaeogeography and correlation of pre-Anglian-Glaciation terraces of the River Thames in Essex and the London Basin. Proceedings of the Geologists' Association, Vol.103, 37-56.

Rose, J. 1994. Major river systems of central and southern Britain during the Early and Middle Pleistocene. Terra Nova, Vol.6, 435-443.